Pedigree Notes

Concept Review• Homozygous: Two of the same allele

– Ex: (TT) homozygous dominant or (tt) homozygous recessive

• Heterozygous: Two different alleles– Tt– One allele must come from the mother and one comes

from the father– The actual allele is decided through haploid gamete

(sperm and egg) formation during meiosis– A zygote is the result of sperm fertilizing an egg

• Autosome: one of the 22 pairs of non-sex chromosomes

• Sex Chromosome: The 23rd pair of chromosomes which may be XX or XY

• Autosomal disorder- a disorder caused by alleles found on one of the first 22 chromosomes

• Pedigree- A chart that shows the relationships within a family

Common Pedigree Symbols

Mode of Inheritance

• Most human genes are inherited in a Mendelian manner. Probabilities can be calculated with a Punnett square.

• We are usually unaware of their existence unless a variant form is present in the population which causes an abnormal (or at least different) phenotype.

• We can follow the inheritance of the abnormal phenotype and deduce whether the variant allele is dominant or recessive.

Autosomal Dominant

• autosomal dominant• A dominant condition is transmitted in

unbroken descent from each generation to the next. There would be no carriers. Individuals that are Homozygous dominant of heterozygous would show the disorder

Homozygous recessive genotype (nn)

Could be NN or Nn

Autosomal Recessive• A recessive trait will only manifest itself when

homozygous. (example: nn)

• If it is a severe condition it will be unlikely that homozygotes will live to reproduce and thus most occurences of the condition will be in matings between two heterozygotes (or carriers).

• Carrier- Has an allele for a recessive trait but does not show the phenotype.

• An autosomal recessive condition may be transmitted through a long line of carriers before, by ill chance two carriers mate. Then there will be a ¼ chance that any child will be affected.

Autosomal Recessive

• An autosomal recessive condition may be transmitted through a long line of carriers before, by ill chance two carriers mate. Then there will be a ¼ chance that any child will be affected.

N n

n

N NN

Nn nn

Nn

These must be nn

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